1. Field of the Invention
This invention relates to the manufacture of boron trichloride from naturally occurring boric acid salts, such as sodium tetraborate.
2. Description of the Prior Art
Boron trichloride previously has been manufactured step-wise, converting boron salts to boron carbides. This process usually proceeds by first converting boron salts to boron oxides and then reacting the oxides with carbon to form a sintered boron carbide mass which thereafter is reacted with chlorine to form boron trichloride. Alternatively, boron trioxide or any other oxygen-containing boron compound has been reacted with carbon at high temperatures to form boron carbide followed by reaction with chlorine to form boron trichloride such as in U.S. Pat. Nos. 2,097,482, 3,019,089 and 3,152,869.
Boron trichloride also has been produced directly from naturally occurring borates (see U.S. Pat. No. 2,997,370) wherein boron-containing ore is admixed with carbon at temperatures above 500.degree. C but below the melting point of the ore and chlorine gas is passed through a fixed bed of heated ore after which the volatile boron trichloride product is recovered. However, an alkaline chloride is produced as a by-product of this reaction which causes difficulties in the handling of such a fixed bed process, causing particle agglomeration and consequent reaction residue removal problems.
Thus, while boric oxide is a far lower cost source of boron than is boron carbide, it is a more expensive source than the naturally occurring boric acid salts such as sodium tetraborate and therefore, a process employing such naturally occurring boric acid salts would be desirous, particularly if the problems associated with fixed or fluidized bed operations can be overcome.